1. Field of the Invention
The invention broadly relates to dampers, and more particularly to dampers which dampen the orbital motion of rotating shafts.
2. Prior Art
Dampers are used to dampen the vibratory motion in a rotating shaft and reduce the amount of unbalanced forces transmitted to a casing or foundation. Squeeze film dampers utilize a small, oil-filled annular gap between a structure which supports the rotating shaft, and a casing which has an inner cylindrical diameter. The orbital motion of the shaft causes relative motion between the structure and the casing, which changes the shape of the gap in a manner which squeezes an oil film within the gap. This builds pressure that results in a damping force. The casing and the structure are usually separated by a centering spring that offsets the static weight of the rotating shaft. In a conventional damper, the spring is comprised of a squirrel cage connecting the casing and the structure. Such a spring is not optimal because the damper, which tends to be very long, must somehow be configured to fit within a very short axial length.
A device referred to as an integral centering spring squeeze film damper provides a solution to the axial space limitation by taking advantage of wire EDM techniques. The casing, the structure, and the centering spring are made from one integral piece of material. This design results in significant cost savings, but requires special material due to the stresses in the spring itself. In addition, the squeeze film annulus is limited in sizes to 0.012 inches and larger due to the practical limitations of the EDM process. Furthermore, the removing of the material and the characteristically thin sections involved often cause distortions which make it difficult to control the size and shape of the oil annulus and thus the relative positions of the structure and the casing. Further, inspection of the integral centering spring is difficult due to the one-piece construction and the narrow annulus. Still further, the highly stressed areas of the spring are recessed deep within the narrow annulus, so that it is impossible to shot peen or inspect it using NDT methods. Also, after extended use, it is often difficult to clean the narrow oil passages due to the limited access. Also, the residual stresses which result from the wire EDM process can cause dislocation of the structure relative to the case. In order to avoid excessive warping, it is often necessary to provide a relaxation period for the stresses to subside before the bore profile is cut. The wire EDM process causes a damper to lose concentricity since the movement of the inner structure during the relaxation period makes it difficult to find the center of the bore. This results in an additional manufacturing step where the outside diameter of damper is machined or ground concentric to the inside diameter. This also makes it difficult to repair these dampers.